1. Field of the Invention
The present invention relates to a wiring board suitably used for semiconductor packages and modules and to a semiconductor package using the wiring board. More particularly the invention relates to a wiring board, which is capable of including various devices, such as semiconductor devices, in high density, and driving these devices at high speed and which has improved reliability, and to a semiconductor package using the wiring board.
2. Description of the Related Art
In recent years, accompanying an increase in the number of terminals, their narrowed pitches, and increased operation speeds in semiconductor devices which result from the trend toward higher performance and greater functions of the devices, there has been growing demand that wiring boards for packaging provided with the semiconductor device attain higher-density finer wiring and higher-speed operation. As an example of conventional wiring boards for packaging which have found wide use, there are built-up printed boards which are a kind of multilayer printed wiring board.
FIG. 1 is a cross-sectional view showing a conventional built-up printed board. As shown in FIG. 1, this conventional built-up board is provided with a base core substrate 73 made of glass epoxy, and a penetrating through hole 71 having a diameter of about 300 μm is drilled in the base core substrate 73. A conductor wiring 72 is formed on both sides of the base core substrate 73, and an interlayer insulating film 75 is provided so as to cover the conductor wiring 72. Via holes 74 are formed in the interlayer insulating film 75 so as to be connected to the conductor wiring 72, and a conductor wiring 76 is provided on the surface of the interlayer insulating film 75 so as to be connected to the conductor wiring 72 through the via holes 74. The printed board may be provided with a multilayer wiring structure by further repeatedly providing interlayer insulating films in which via holes are formed and conductor wirings on the conductor wiring 76 as required.
However, the built-up printed board has an insufficient heat resistance due to the use of a glass epoxy printed board as the base core substrate 73, so that there is a problem deformations occur with the base core substrate 73, such as shrinkage, warpage and waviness through heat treatment performed for forming the interlayer insulating film 75. As a result, there exists a step of exposing a resist during the formation of the conductor wiring 76 through the patterning of a conductor layer (not shown), the positional accuracy of the exposure is significantly decreased, and hence it becomes difficult to form a high-density and fine wiring pattern on the interlayer insulating film 75. Further, it is necessary to provide land portions at the junctions of the conductor wiring 72 and the penetrating through holes 71 to securely connect them. Even though a wiring having provision for high-speed operation is designed on the built-up layer having the interlayer insulating film 75 and the conductor wiring 76, the control of impedance becomes difficult and loop inductance becomes high due to the presence of the land portions. Because of this, the operating speed of the entire built-up printed board is reduced, which brings about a problem that it is difficult to make the provision for high-speed operation.
To solve these problems caused by the penetrating through holes of the built-up printed board, a method for forming a printed board has been described in place of the method for drilling the penetrating through holes in a glass epoxy substrate in, for example, Japanese Patent Publication Laid-Open No. 2000-269647 and a reference “Proceedings For The Eleventh Microelectronics Symposium, pp. 131 to 134.”
FIG. 2A to 2C are cross-sectional views showing this conventional printed board in the order of its manufacturing steps. To begin with, a prepreg 82, on which a predetermined conductor wiring 81 is formed, is prepared as shown in FIG. 2A. Then through holes 83 having a diameter of 150 to 200 μm are formed in the prepreg 82 by means of laser processing. Further, as shown in FIG. 2B, the through holes 83 are filled with a conductor paste 84. Still further, as shown in FIG. 2C, such a prepreg 82, that is, the prepreg 82 having the through holes 83 filled with the conductor paste 84 is fabricated plurally, and then the prepregs 82 thus fabricated are laminated together. At this time, the land pattern 86 of the conductor wiring 81 is connected to the through holes 83 of the adjacent prepreg, which allows a printed board 85 having no penetrating through hole to be fabricated.
However, in this conventional art, a positional accuracy in the lamination of the prepregs 82 is low, which brings about a problem that it is difficult to reduce the diameter of the land pattern 86. Therefore, it is difficult to implement high-density wiring, and the effects of improving the controllability of the impedance and of decreasing the loop inductance are insufficient. Furthermore, there is a problem that the reliability of the through hole connectivity after the lamination is poor.
To solve the number of problems, the inventors have developed a method for fabricating a wiring board through the formation of a wiring layer on a supporting structure such as a metal sheet and the subsequent removal of the supporting structure. The method is disclosed in Japanese Patent Publication Laid-Open No. 2002-198462 (page 8 and 11 and FIG. 17).
FIGS. 3A and 3B are cross-sectional views showing this conventional wiring board illustrated in the order of its manufacturing steps. To begin with, a supporting sheet 91 made of a metal sheet or the like is prepared as shown in FIG. 3A. A conductor wiring 92 is formed on the supporting sheet 91, an interlayer insulating film 93 is formed so as to cover the conductor wiring 92, and via holes 94 are formed in the interlayer insulating film 93 so as to be connected to the conductor wiring 92. Thereafter, a conductor wiring 95 is formed on the interlayer insulating film 93. The conductor wiring 95 is formed so as to be connected to the conductor wiring 92 through the via holes 94. By repeating the steps of forming the interlayer insulating film 93, the via holes 94, and the conductor wiring 95 as required, a multilayer wiring may be implemented. Then, as shown in FIG. 3B, a part of the supporting sheet 91 is removed by etching to expose the conductor wiring 92 and form a supporting structure 96, by which a wiring board 97 is fabricated.
At this time, as the interlayer insulating film 93, a single-layer film made of an insulating material, which has a film strength 70 MPa or more, an elongation percentage after breaking of 5% or more, a glass-transition temperature of 150° C. or more, and a coefficient of thermal expansion of 60 ppm or less, or a single-layer film made of an insulating material having an elastic modulus of 10 GPa or more, a coefficient of thermal expansion of 30 ppm or less, and a glass-transition temperature of 150° C. or more is used.
According to the technique, since the wiring board 97 has no penetrating through hole, the problems caused by the penetrating through holes can be solved, which allows a high-speed transmission to be designed. Also, since a high-heat-resistant metal sheet or the like is used as the supporting sheet 91, deformations, such as shrinkage, warpage, and waviness, do not occur in contrast to the case where the glass epoxy substrate is used, which makes it possible to implement a high-density fine wiring. Further, a wiring board having high strength can be obtained by determining the mechanical characteristics of the interlayer insulating film 93 as described above.
However, the conventional art has a problem described below. The wiring board 97 shown in FIG. 3B is extremely thin because of the absence of a base core substrate, but the wiring board 97 is capable of achieving a sufficient strength when fabricated initially through the determination of the mechanical characteristics of the interlayer insulating film 93 described above.
However, the wiring board 97 is generally provided with a large-area semiconductor device to form a semiconductor package, and then the semiconductor package is mounted to a mounting board such as a printed board. The semiconductor device generates heat during operation to be raised in temperature and ceases the heat during quiescent operation to be lowered in temperature. Because of this, during the semiconductor device operation, a thermal stress is applied to the wiring board 97 due to a difference in the coefficients of thermal expansion of the semiconductor device and the mounting board. Therefore, when the semiconductor device is operated repeatedly in a state that the semiconductor device is mounted to the wiring board 97 as described above, the thermal stress is repeatedly applied to the wiring board 97, so that cracks may occur in the interlayer insulating film 93 etc., of the wiring board 97. Because of this, there is a problem that it is impossible to secure reliability required for the wiring board and the semiconductor packages.